1. Field of the Invention
The present invention pertains to an image forming apparatus that performs image stabilizing control, and more particularly, to image stabilizing control performed when the operation of the image forming apparatus is restored after it is suspended.
2. Description of the Related Art
Image stabilizing control to maintain high image quality for recorded images is performed in image forming apparatuses in general. For example, in image forming apparatuses using the electrophotographic method, a test toner image (patch) is formed on the photoreceptor and its density is detected and compared with an expected value. Based on the result of the comparison, the charger output to charge the surface of the photoreceptor and the developing bias voltage to be applied are adjusted. These operations are done in order to prevent the image quality from deteriorating due to the change in photoreceptor characteristics that is caused by an environmental change. The reference density value is the toner density that is expected from the charger output when forming the test toner image and the applied developing bias voltage (these may be called the image stabilizing parameters). Image stabilizing control to optimize these parameters is performed when the main switch is turned ON, before and after image formation, and when the apparatus returns to operation after operation is suspended due to some sort of failure, for example. Such control is particularly needed when recording color images or images as to which accurate gradation reproduction is required.
U.S. Pat. No. 5,387,965 discloses a control method in which the density of the test toner image formed on the photoreceptor is compared with the expected value and the target toner density value (target value during toner supply) in the developing device is corrected based on the result of the comparison. Through this control, the response to changes in the photoreceptor characteristics or changes in the developer characteristic that occur over time can be improved.
This image stabilizing control requires a certain period of time. For example, in an image forming apparatus using the electrophotographic method, a certain period of time is needed for the formation of a test toner image, the detection of the density of the test toner image, the setting of the voltage to charge the photoreceptor, and the setting of the developing bias voltage. Consequently, when the apparatus returns to operation after operation is suspended, the commencement of the image forming operation is delayed by the amount of time needed for performance of the image stabilizing control.
Suspension of operation of an image forming apparatus is caused due to some type of failure. The failure may be one that requires only a short suspension period before recovery, such as a recording paper jam, in which the problem is eliminated simply by removing the jammed paper. In such a case, the environmental factors (i.e., temperature and humidity) that affect the image stabilizing parameters do not change significantly during the time that operation is suspended. In many cases, good images can be formed even if the parameter values immediately before the suspension of operation are used in the image formation. If image stabilizing control is carried out in such cases, the commencement of the image forming operation is needlessly delayed.
On the other hand, where it takes a long time to remove the jammed paper or where the image forming apparatus is not used for a long time before or after the jammed paper is removed, the environmental values may change substantially during the time that operation is suspended. In such cases, image stabilizing control is essential even if the failure is simply a paper jam. In addition, some of the failures that cause suspension of the operation of the image forming apparatus consist of failures of the elements that directly affect image quality. Image stabilizing control is imperative in such a case as well.